This isn't exactly the quickest way, but you could search by file types first, which might help you narrow down large e-mails. For instance, you could do a search for video file types (which are huge) first, like this:

I tried with a throw-away account, and it works as advertised... however I think FindBigMail.com is more secure and usable (you can check the contents of the mail before deleting.)
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LeftiumNov 10 '10 at 17:42

FindBigMail will label all your large quota-killing messages. Just click the various labels to show messages that are greater than the size indicated by the label.

... or this is a very round-about way, but you can:

Download your emails with Gmail Backup. Each individual email is downloaded as an EML file. Many email programs can read EML files; they are just plain text.

Prune out all the large EML files. You can sort all the EML files by size.

Restore the backup to Gmail. You will probably need to delete the messages from Gmail, first.

I am using Gmail Backup to migrate my mail to a new Gmail account. Also, I am fixing the timezone on several emails. (They got the wrong timezone because I imported an Outlook IMAP store from a computer with a different timezone.)

Someone has worked out a way to do this with Google Docs. The beauty of this method is that you don't need to rely on a third-party; everything stays within the Google ecosystem.

The idea is that your Google Docs will connect to your Gmail account and compute the size of every message that’s present in your mailbox. If it finds a bulky message (size > 1 MB), it will make a note of it in the spreadsheet.

Once the sheet has a list of all the bulky message, you can sort the sheet by the Size column to find the big ones. Or use the Filter option (the Funnel icon) to find messages that are within a particular range (5 MB < size < 10 MB). Click the “View” link to open the corresponding message in Gmail, forward it to a secondary email address and delete it from the primary Inbox to recover space.

That’s all the theory you should know, let’s now put this program into action:

Once the permissions have been granted, choose “Scan Mailbox” from the Gmail menu to start the scanning process.

Sit back and relax as the last step may take time depending on how big your Gmail mailbox is. Also, if the program is stuck or if you accidentally close the browser tab, open the same Google sheet, choose “Scan Mailbox” again and the script will resume scanning from where it left off.

When I had to search for a large e-mail, I went for the try-and-see-if-it-works method, and used the search term size:5000000 to find e-mail of 5MB large, and that seemed to actually find all my emails larger than 5MB. I tried different numbers and it seemed to work consistently.

Even though this operator isn't documented in the advanced search options, it worked for me. :)